A variety of implantable medical devices (IMD's) exist that provide diagnostic or therapeutic capabilities. These IMD's include, for example, cardiac pacemakers, implantable cardioverters/defibrillators (ICD's), and various tissue, organ and nerve stimulators or sensors. IMD's typically include their components within a hermetically sealed enclosure referred to as a “can” or housing. In some IMD's, a connector header or connector block is attached to the housing and allows interconnection with one or more elongated electrical medical leads.
The header is typically molded from of a relatively hard, dielectric, non-conductive polymer such as tecothane having a thickness approximating the housing thickness. The header includes a mounting surface that conforms to and is mechanically affixed against a surface of the housing.
It has become common to provide a communication link between the hermetically enclosed electronic circuitry of the IMD and an external programmer or monitor or other external medical device (EMD) in order to provide for downlink telemetry (DT) transmission of commands from the external device to the IMD and to allow for uplink telemetry (UT) transmission of stored information and/or sensed physiological parameters from the IMD to the EMD.
As the technology has advanced, IMDs have become ever more complex in possible programmable operating modes, menus of available operating parameters, and capabilities of monitoring increasing varieties of physiologic conditions and electrical signals which place ever increasing demands on the programming system. Additionally, the technology advancements have resulted in a reduction in the physical size of most components utilized in IMDs and thus the physical size of the devices—including the housing—has continually decreased as well. Conventionally, the communication link between the IMD and the EMD is by encoded RF transmissions between an IMD RF telemetry antenna and transceiver and an EMD RF telemetry antenna and transceiver. Generally, the antenna is disposed within the header of the IMD. The reduced overall size of the IMD has resulted in restricted space allocations for an IMD RF telemetry antenna.
“Far field” telemetry, or telemetry over distances beyond the near field region for an IMD is desirable. Various proposals have been advanced to provide an IMD with an antenna that facilitates far field telemetry. The proposals include eliminating the ferrite core, wire coil, RF telemetry antenna, utilizing alternative IMD telemetry antennas, and substituting alternative telemetry transmission systems and schemes employing far higher carrier frequencies and more complex signal coding to enhance the telemetry transmission distances to allow telemetry transmission to take place over a matter of meters rather than inches. These approaches are generally undesirable in that depending upon the option selected they require additional components added to the housing, reduce the effectiveness of other components, create a directional requirement, add extraneous exposed components or require additional considerations during implant, or that they are simply infeasible due to space constraints.
To implement effective telemetry from a given IMD over the distances desired, the driving power should be efficiently converted to maximize the far-field component generated by the antenna. One factor affecting the far field component is the length of the antenna with respect to the wavelength of the driving signal. While many types of antennas function according to a variety of parameters, it is generally desirable to provide an antenna having a minimum length equivalent to one-quarter or one-half the wavelength of the driving frequency. However, due to the physical space limitations it has become increasingly difficult to provide a miniaturized antenna that also meets the desired minimum length of one-quarter or one-half the wavelength. Further, as the antenna length decreases the radiation resistance of the antenna diminishes, making it more difficult to couple significant power to the antenna thereby decreasing the performance of the antenna. It remains desirable to provide an IMD telemetry antenna with improved performance.
The disclosure will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings which are illustrative in nature and non-limiting. Various modifications are permissible while remaining within the spirit and scope of the disclosure as provided in the attached claims.